Synthetic leather or imitation leather is a woven or non-woven textile that is impregnated with a polymer such as polyurethane that may have a porous polymer coating (poromeric) layer thereon.
Synthetic leather is typically made by impregnating non-woven textiles with polyurethane to bond the material and give it the mechanical properties and feel (hand) similar to real leather. Generally, synthetic leather is made using an organic solvent by a wet coagulation or dry coagulation process. In the wet coagulation process, the textile is impregnated with a polyurethane dissolved in a volatile organic solvent such as dimethylformamide (DMF) and the polyurethane is coagulated in a non-solvent such as water, and the solvent is extracted by the water. In the dry coagulation process, the textile is impregnated, for example, with polyurethane dissolved in an organic solvent and impregnated textile is subsequently dried. Because of the organic solvent, a porous flexible structure is developed upon coagulation resulting in a flexible leatherlike material.
These methods, even though they give a useful synthetic leather, require excessive amounts of volatile organic solvents, which are released to the environment or require expensive recovery systems. In addition, because the removal and distribution of the solvent that causes the porous structure is difficult to control, the resultant synthetic layer typically does not have a well defined porous structure leading to variations of the synthetic leather.
To remedy these problems, attempts have been made to replace the solvent based processes using aqueous polyurethane dispersions to impregnate the textile and make the porous coating layer when desired. Early examples such as U.S. Pat. Nos. 4,171,391 and 4,376,148 describe internally stabilized polyurethane dispersions (e.g., anionic internally stabilized using 2,2-di-(hydroxymethyl) propionic acid) impregnated into a textile. These dispersions were coagulated using a weak acid such as acetic acid to avoid contamination and unsatisfactory coagulation. Consequently, the coagulation times were long, for example, 5 to 10 minutes. The synthetic leather that was formed was stiff resembling cellulose cardboard. Externally stabilized polyurethane dispersions were avoided because of the need to use large amounts of surfactant, which were deleterious to the synthetic leather.
Another example, U.S. Pat. No. 4,496,624, describes anionic internally stabilized polyurethane dispersions blended with other polymeric dispersions (e.g., vinylchoride/vinylidene chloride copolymer) impregnated into textiles and coagulated using sodium silicofluoride and hot water (e.g., 200° F.). The impregnated sheet was then dried. The dried impregnated sheet was boardy. The dried sheet was then pressed at an elevated temperature (e.g., 275° F.). The heated and pressed sheet was soft and pliable.
A recent example, U.S. Pat. No. 6,231,926, also describes impregnating a textile with an internally stabilized aqueous polyurethane dispersion until the textile is completely impregnated. The impregnated textile is dried. The dried impregnated textile is subjected to a caustic solution to remove some of the polyurethane impregnated into the textile to achieve a satisfactory hand.
Another recent example, WO 02/33001, describes an anionic internally stabilized polyurethane impregnated into a textile and formation of a porous layer. The method requires an antifoam and water repellant for the impregnating dispersion. Coagulation time was 5 minutes or more.
Accordingly, it would be desirable to provide a synthetic leather and method to form the synthetic leather that avoids one or more of the problems in the prior art such as one of those described above (e.g., use of organic solvents, slow coagulation times, use of hazardous or caustic chemicals to coagulate, use of expensive additives and extra processing steps such as caustic leaching).